Stateless Network Address Translation 64

The Stateless
Network Address Translation 64 (NAT64) feature provides a translation mechanism
that translates an IPv6 packet into an IPv4 packet and vice versa. The
translation involves parsing the entire IPv6 header, including the extension
headers, and extracting the relevant information and translating it into an
IPv4 header. Similarly, the IPv4 header is parsed in its entirety, including
the IPv4 options, to construct an IPv6 header. This processing happens on a
per-packet basis on the interfaces that are configured for Stateless NAT64
translation.

The Stateless NAT64
translator does not maintain any state information in the datapath. This
translator is based on the IETF working group Behavior Engineering for
Hindrance Avoidance (BEHAVE) drafts about the framework for IPv4/IPv6
translation. This draft describes the mechanism to translate an IPv6 packet to
an IPv4 packet and vice versa, including the transport layer headers and
Internet Control Message Protocol (ICMP).

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information,
see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module,
and to see a list of the releases in which each feature is supported, see the feature information table.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature
Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Restrictions for Stateless
Network Address Translation 64

The following
restrictions apply to the Stateless NAT64 feature:

Only valid
IPv4-translatable addresses can be used for stateless translation.

Multicast is
not supported.

Applications
without a corresponding application layer gateway (ALG) may not work properly
with the Stateless NAT64 translator.

Fragmentation of IP
Datagrams in IPv6 and IPv4 Networks

In IPv4 networks,
any intermediate router can do the fragmentation of an IP datagram. However, in
IPv6 networks, fragmentation can be done only by the originating IPv6 host.
Because fragmentation in IPv6 networks is done by the IPv6 hosts, the path
maximum transmission unit (PMTU) discovery should also be done by the IPv6
hosts. However, a PMTU discovery is not possible across an IPv4 network where
the routers are allowed to fragment the packets. In IPv4 networks, a Stateless
NAT64 translator is used to fragment the IPv6 datagram and set the Don’t
Fragment (DF) bits in the IPv4 header. Similarly, the translator can add the
fragment header to the IPv6 packet if an IPv4 fragment is received.

Translation of ICMP for
Stateless NAT64 Translation

The IETF draft on
the IP/ICMP translation algorithm describes the ICMP types or codes that should
be translated between IPv4 and IPv6. ICMP errors embed the actual IP header and
the transport header. Because the ICMP errors are embedded in the IP header,
the IP header is not translated properly. For ICMP error packets, Stateless
NAT64 translation should be applied twice: once for the outer header, and once
again for the embedded header.

IPv4-Translatable IPv6
Address

IPv4-translatable
IPv6 addresses are IPv6 addresses assigned to the IPv6 nodes for use with
stateless translation. IPv4-translatable addresses consist of a variable-length
prefix, an embedded IPv4 address, fixed universal bits (u-bits), and in some
cases a suffix. IPv4-embedded IPv6 addresses are IPv6 addresses in which 32
bits contain an IPv4 address. This format is the same for both IPv4-converted
and IPv4-translatable IPv6 addresses.

The figure below
shows an IPv4-translatable IPv6 address format with several different prefixes
and embedded IPv4 address positions.

Prefixes Format

A set of bits at
the start of an IPv6 address is called the format prefix. Prefix length is a
decimal value that specifies how many of the leftmost contiguous bits of an
address comprise the prefix.

An embedded IPv4
address is used to construct IPv4 addresses from the IPv6 packet. The Stateless
NAT64 translator has to derive the IPv4 addresses that are embedded in the
IPv6-translatable address by using the prefix length. The translator has to
construct an IPv6-translatable address based on the prefix and prefix length
and embed the IPv4 address based on the algorithm.

According to the
IETF address format BEHAVE draft, a u-bit (bit 70) defined in the IPv6
architecture should be set to zero. For more information on the u-bit usage,
see RFC 2464. The reserved octet, also called u-octet, is reserved for
compatibility with the host identifier format defined in the IPv6 addressing
architecture. When constructing an IPv6 packet, the translator has to make sure
that the u-bits are not tampered with and are set to the value suggested by RFC
2373. The suffix will be set to all zeros by the translator. IETF recommends
that the 8 bits of the u-octet (bit range 64-71) should be set to zero.

The prefix lengths
of 32, 40, 48, 56, 64, or 96 are supported for Stateless NAT64 translation. The
Well Known Prefix (WKP) is not supported. When traffic flows from the
IPv4-to-IPv6 direction, either a WKP or a configured prefix can be added only
in stateful translation.

Supported Stateless NAT64
Scenarios

The IETF framework
draft for IPv4/IPv6 translation describes eight different network communication
scenarios for Stateless NAT64 translation. The following scenarios are
supported by the Cisco IOS Stateless NAT64 feature and are described in this
section:

Scenario 1--an
IPv6 network to the IPv4 Internet

Scenario 2--the
IPv4 Internet to an IPv6 network

Scenario 5--an
IPv6 network to an IPv4 network

Scenario 6--an
IPv4 network to an IPv6 network

The figure below
shows stateless translation for scenarios 1 and 2. An IPv6-only network
communicates with the IPv4 Internet.

Figure 2. Stateless Translation for
Scenarios 1 and 2

Scenario 1 is an
IPv6 initiated connection and scenario 2 is an IPv4 initiated connection.
Stateless NAT64 translates these two scenarios only if the IPv6 addresses are
IPv4 translatable. In these two scenarios, the Stateless NAT64 feature does not
help with IPv4 address depletion, because each IPv6 host that communicates with
the IPv4 Internet is a globally routable IPv4 address. This consumption is
similar to the IPv4 consumption rate as a dual-stack. The savings, however, is
that the internal network is 100 percent IPv6, which eases management (Access
Control Lists, routing tables), and IPv4 exists only at the edge where the
Stateless translators live.

The figure below
shows stateless translation for scenarios 5 and 6. The IPv4 network and IPv6
network are within the same organization.

Figure 3. Stateless Translation for
Scenarios 5 and 6

The IPv4 addresses
used are either public IPv4 addresses or RFC 1918 addresses. The IPv6 addresses
used are either public IPv6 addresses or Unique Local Addresses (ULAs).

Both these
scenarios consist of an IPv6 network that communicates with an IPv4 network.
Scenario 5 is an IPv6 initiated connection and scenario 6 is an IPv4 initiated
connection. The IPv4 and IPv6 addresses may not be public addresses. These
scenarios are similar to the scenarios 1 and 2. The Stateless NAT64 feature
supports these scenarios if the IPv6 addresses are IPv4 translatable.

Multiple Prefixes Support for Stateless NAT64 Translation

Network topologies that use the same IPv6 prefix for source and destination addresses may not handle routing correctly and
may be difficult to troubleshoot. The Stateless NAT64 feature addresses these challenges in Cisco IOS XE Release 3.3S and
later releases through the support of multiple prefixes for stateless translation. The entire IPv4 Internet is represented
as using a different prefix from the one used for the IPv6 network.

Additional References for
Stateless Network Address Translation 64

Related Documents

Standards and RFCs

Standard/RFC

Document Title

RFC 6052

IPv6 Addressing of IPv4/IPv6 Translators

RFC 6144

Framework for IPv4/IPv6 Translation

RFC 6145

IP/ICMP Translation Algorithm

Technical Assistance

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Feature Information for Stateless Network Address Translation 64

The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

The Stateless Network Address Translation 64 feature provides a translation mechanism that translates an IPv6 packet into
an IPv4 packet and vice versa. The translation involves parsing the entire IPv6 header, including the extension headers, and
extracting the relevant information and translating it into an IPv4 header. Similarly, the IPv4 header is parsed in its entirety,
including the IPv4 options, to construct an IPv6 header. This processing happens on a per-packet basis on the interfaces that
are configured for Stateless NAT64 translation.

In Cisco IOS XE Release 3.10S, support was added for the Cisco ISR 4400
Series Routers.

Glossary

ALG —application-layer gateway or application-level gateway.

FP —Forward Processor.

IPv4-converted address —IPv6 addresses used to represent the IPv4 hosts. These have an explicit mapping relationship to the IPv4 addresses. This
relationship is self-described by mapping the IPv4 address in the IPv6 address. Both stateless and stateful translators use
IPv4-converted IPv6 addresses to represent the IPv4 hosts.

IPv6-converted address —IPv6 addresses that are assigned to the IPv6 hosts for the stateless translator. These IPv6-converted addresses have an explicit
mapping relationship to the IPv4 addresses. This relationship is self-described by mapping the IPv4 address in the IPv6 address.
The stateless translator uses the corresponding IPv4 addresses to represent the IPv6 hosts. The stateful translator does not
use IPv6-converted addresses, because the IPv6 hosts are represented by the IPv4 address pool in the translator via dynamic
states.

NAT —Network Address Translation.

RP —Route Processor.

stateful translation —In stateful translation a per-flow state is created when the first packet in a flow is received. A translation algorithm
is said to be stateful if the transmission or reception of a packet creates or modifies a data structure in the relevant network
element. Stateful translation allows the use of multiple translators interchangeably and also some level of scalability. Stateful
translation is defined to enable the IPv6 clients and peers without mapped IPv4 addresses to connect to the IPv4-only servers
and peers.

stateless translation —A translation algorithm that is not stateful is called stateless. A stateless translation requires configuring a static translation
table, or may derive information algorithmically from the messages it is translating. Stateless translation requires less
computational overhead than stateful translation. It also requires less memory to maintain the state, because the translation
tables and the associated methods and processes exist in a stateful algorithm and do not exist in a stateless one. Stateless
translation enables the IPv4-only clients and peers to initiate connections to the IPv6-only servers or peers that are equipped
with IPv4-embedded IPv6 addresses. It also enables scalable coordination of IPv4-only stub networks or ISP IPv6-only networks.
Because the source port in an IPv6-to-IPv4 translation may have to be changed to provide adequate flow identification, the
source port in the IPv4-to-IPv6 direction need not be changed.